As known to those skilled in the art, centralizers are used in the oil, gas or water well drilling industries to centre a tubular member (hereinafter referred to as “tubular”) within a borehole or previously installed larger tubular.
Such tubulars are generally constructed in handleable lengths, e.g. 12 m (40 ft), each length typically being externally male threaded at both ends. The lengths are assembled together using short female threaded couplings. The assembly of the tubulars to a predetermined total length is referred to as a ‘string’.
When the string is disposed in a borehole or existing tubular, it is desirable to position the string substantially centrally within the borehole or existing tubular thereby forming a substantially annular passageway around the tubular of concern. This enables passage of material such as fluids, cement slurries in the space around the tubular.
To try to achieve this condition, centralizers are disposed at selected intervals along the length of the string. Retention of the centralizers in a desired position may be achieved in restricting axial movement by the use of a so-called “stop collar” being a ring grippingly secured to the tubular. The stop collar design must cope with free fitment onto tubulars having poorly toleranced outer diameters. Any design applied must take up this tolerance as pre-requisite to applying sufficient load to give the desired axial load restraint. To resist axial loading, the stop collar may have, for example, toughened steel screws radially dispersed around the circumference of the stop collar that protrude substantially above the outer surface of the stop collar body.
Known spring centralizers have a flexible external diameter aimed at making contact with the bore wall at all times while being capable of flexing to react (restoring force), the lateral forces created by the tubular conforming to the wellbore profile and accommodate obstructions or internal dimensional changes. Such centralizers are comprised of circular end bands between which are affixed a number of leaf springs commonly referred to as ‘bows’.
It is desired within the industry that the centrality of the tubular as it is being moved down the borehole to its required final depth position is sufficient to keep the tubular from contacting the borehole or tubular bore such that undue mechanical interference and damage is avoided, be it from the centralizer, stop collar protrusions or couplings.
Contact forces, e.g. stop collar screws, can cause considerable damage to the previously installed steel tubular and generated swarf may cause damage elsewhere in the overall well construction. Too much deflection of centralizer bows will permit contact of, e.g. stop collar screws, which are affixed to the rotating tubular, cutting into the wellbore or larger tubular to which the centralizer is being inserted.
Commonly, especially in well remedial work, the previously installed tubular may have what is referred to as a ‘Window’ cut through the side of the tubular to permit the centralized tubular being run to deflect through the window. It follows that, for example, hardened stop collar protrusions and couplings could hold fast against a window edge if lateral forces of deflected centralizer bows are equal to or below the annular height of the protrusions. Hence, in such arrangement it can be quite problematic to move centralizer through the bore hole and into position.
It is furthermore imperative to facilitate the common practice of rotating the tubular as it passes through to its final depth thereby easing passage through high Dog Leg Severity (DLS) undulations. Centralizer rotation is stopped against the bore through which it is being run permitting rotation of the tubular inside the affixed centralizers.
Evolution of wellbore profile complexity has exacerbated occurrence of such mechanical interference. Undulations of the profile defined as a rate of 3 dimensional change referred to as DLS (Dog Leg Severity) per unit length of bore, commonly 30 m (100 ft) frequently result in high lateral forces, perpendicular to the tubular axis. These lateral forces can be such that centralizer spring bows may become flattened or near flattened at various points of high DLS during passage of the centralizer down a wellbore. This can result in, for example, couplings and stop collar protrusions, such as hardened set screws running against the previously installed tubular bore or wellbore. In other words mechanical interference between these parts of the tubular and the previously installed tubular bore or wellbore can occur, leading to surface damage to the previously installed tubular bore or wellbore.
It is additionally noted that said flattening of bows may result in permanent deformation of the bows, especially at the point of spring rotation at the meeting point of leaf, or bow, spring to end band. This can result in the original centralization potential of the centralizer becoming adversely affected when a desired depth is reached. It is preferred within the industry that centrality between tubulars or tubular and the wellbore, when centralized tubular is at its required depth, is maximised or to a minimum acceptable level. In other words, the tubular is located centrally within the previously installed tubular or wellbore, or the distance between the tubular and previously installed tubular or wellbore is maintained above a minimum distance.
FIG. 1 illustrates a known tubular arrangement that has been inserted within a borehole 50. A centralizer 38 is located on the tubular by way of stop collars 37 located either side of the centralizer 38. Stop collars 37 are used to mount around the tubular to engage and grip the exterior of the tubular. The stop collars 37 provide a stop shoulder on the tubular to restrict axial travel along the tubular member of any further associated product such as a centralizer 38. Each centralizer 38 is therefore joined to the tubular and arranged to support the tubular within the borehole 50 such that the tubular is substantially centrally arranged within the borehole 50.
The one-piece centralizer 38 has first and second opposing end collars 41, 42 that are axially separated by plural spring bows. Only spring bows 43, 44, 45, and 46 of the plural spring bows are shown.
Each spring bow forms a generally convex curve. This is clearly observed for spring bows 45 and 46. However, the effect of high lateral forces from the tubular has caused deflection of the centralizer spring bows. This has caused the flattening of spring bow 43. The lateral forces have caused sufficient deflection for some of the set screws 47 of the stop collar 37 to be pushed hard against the borehole 50.
This situation can be problematic. This is because the tubular is now contacting the borehole 50, which can lead to mechanical interference and damage. Furthermore, contact forces from, for example, the contact screws 47, can cause damage to the borehole 50. Too much deflection of the centralizer spring bows will enable, for example, stop collar screws 47 to cut into the well bore 50. Considerable damage to the borehole 50 can occur. This damage can also generate swarf that can cause damage elsewhere. Due to flattening of the spring bow 43, parts of the tubular, for example the stop collar screws 47 of stop collar 37 that is grippingly attached to the tubular, can hold fast to the borehole 50 because the stop collar 37 is pushed against the borehole 50. This can constrain the tubular from rotating. This situation also applies for a tubular inserted within a previously installed tubular rather than within a borehole 50.
Additionally, spring bow 43 has been flattened to an extent that can cause permanent or irreversible deformation. Spring bow 43, and any other spring bow that has similarly been flattened, will not now spring back to its original shape or not spring back sufficiently to provide the required centring or restraining effect. This means that centralizer 38 cannot now centralize the tubular optimally. This can occur, for example, when the tubular is further passing through the borehole 50 or tubular bore 40. When there are high lateral forces, because the spring bow 43 is now flattened, the centralizer 38 will be offset, or can be more easily offset, and it becomes more likely that mechanical interference between the tubular and the bore hole 50 or tubular bore 40 will result. This is because, since spring bow 43 has become flattened, the centralizer 38 cannot perform its function correctly and centre the tubular within the borehole 50 or tubular bore 40. Therefore, even lateral forces less than that that were required to flatten spring bow 43 can now push the tubular hard against the borehole 50 or tubular bore because spring bow 43 is flattened and cannot resist lateral forces. This pushing of the tubular against the borehole 50 or tubular bore 40 can cause damage as already discussed. Also, when the tubular reaches its final depth, centralizer 38 can be located at a position where the borehole 50 has a wider diameter than that typical for other parts of the borehole 50. Such a scenario is with so-called “under reamed” bores, occurring where wellbores are ‘opened out’ in a region lower than a previously installed tubular. In such a circumstance, because spring bow 43, and indeed other spring bows, of the centralizer 38 has or have been flattened, leading to permanent deformation or otherwise to the spring bows not functioning optimally, the centralizer 38 cannot mechanically secure the tubular in position at that location. This is because the flattened spring bow 43, and indeed other flattened spring bows, will not spring back to their original shape, or will not spring back sufficiently to make the required contact with the wall of the borehole 50 or tubular bore 40. Therefore, the centralizer will not make the required robust mechanical connection with the borehole 50 at that location, and the tubular will neither be centrally located, nor mechanical constrained to the required degree, within the borehole 50 or tubular bore 40.
A problem with existing centralizers that are installed on a tubular inserted within a borehole or previously installed tubular, is that damage can occur to the wall of the borehole or previously installed tubular. This damage is due to mechanical interference occurring between, for example the screws on stop collars, and the wall of the borehole or previously installed tubular, which is further exacerbated by the flattening of the spring bows.